The present invention relates to a method for measuring the thickness of a magnetic recording medium, and particularly to a method for measuring the thickness of a multi-layer film of a magnetic recording medium formed using a simultaneous stratification coating process.
In a magnetic recording medium such as a magnetic tape, magnetic disk, or the like, the number of layers contained within the magnetic recording layer has increased from a single layer to multiple layers in order to increase the recording/reproducing density of the recording layer. For producing such a magnetic recording medium having multiple magnetic recording layers, there are known a process of repeating coating and drying for every layer so as to form multiple recording layers, and a process for forming multiple magnetic recording layers simultaneously with each other. Examples of the latter process are disclosed in Japanese Unexamined Patent Publications (OPIs) Nos. 48-98803, 48-99233, and 62-124631.
In the former process, the thicknesses of each of the layers can readily be determined by carrying out a measurement every time the coating of one layer has been performed. In the latter process (simultaneous stratification coating process), however, it has been very difficult to measure the thicknesses of each of the layers. Conventionally, first the thickness of a sample is measured after only a lower layer has been coated onto the support. Then, the coating of two layers is performed simultaneously, and the total thickness of the two layers is measured and the thickness of the upper layer calculated by subtracting the thickness of the lower layer from the total thickness of the two layers.
In this conventional thickness measurement method, however, there has been a problem in that on-line thickness control cannot be performed during manufacture, and also in that it takes a very long time in the case where the thickness is set at the start (at the initiation of the coating operation) or in the case where the thickness of an upper or lower layer is changed. Further, the measurement process results in production losses, and the measurement is poor in accuracy.
In recording on a magnetic tape, generally the video signal is recorded in the magnetic recording layer close to the surface thereof, while the audio signal is recorded in the whole magnetic recording layer. Therefore, for the production of a video tape having a multi-layer structure, not only is it very important to measure and control the thickness and uniformity of thickness of the uppermost layer, but it is also very important to be able to accurately measure the thickness of the lower layer to thereby measure and control the thickness of the overall recording layer.
In conventional thickness measurement methods such as a method in which a measured amount of transmission or absorption of visible rays or infrared rays is converted into thickness, a method in which the quantity of a designated element (mainly Fe in the case of a magnetic recording material layer) is measured using fluorescent X-rays and converted into thickness, or a method using a contact thickness gauge, however, there has been a problem in that the individual thicknesses of each layer cannot be measured, although the total thickness of the layers can be measured.